CSS Grid is revolutionizing web design. It’s a flexible, simple design standard that can be used across all browsers and devices. Designers and developers are rapidly falling in love with it and so are we. That’s why we’ve been working hard on the Firefox Developer Tools Layout panel, adding powerful upgrades to the CSS Grid Inspector and Box Model. The latest improvements are now available in Firefox Nightly.

Source: Powerful New Additions to the CSS Grid Inspector in Firefox Nightly